Power transmission from a source to a load typically requires multiple power conversion stages such as from high voltage AC source to a lower voltage AC at the loads. Many modern power systems require large and heavy conventional transformers. The weight and volume of these transformers is a barrier to the development of expanded electrical capabilities associated with certain power system applications.
This is especially relevant with the 50 Hz and 60 Hz transformers used for many utility distribution systems as well as the electrical distribution systems associated with ships, planes, and railroads. The size of the present conversion systems impacts the placement criteria that can be problematic in space-restricted areas. The weight of the transformers not only impacts the logistics, but also impacts the operational efficiency if the heavy transformers are carried onboard.
Recent developments and designs indicate that high frequency solid state or electronic transformers are enhanced replacements for bulky line-frequency iron core transformers. High frequency switched power electronic transformers bring about significant reductions in size and weight compared to conventional line-frequency transformers. In addition, controls can be embedded to provide enhanced functionality such as fault current limiting and improved power quality.
One known approach uses rectification of the incoming AC source to a DC signal followed by a high frequency isolated DC-DC converter and then inversion back to the output AC signal. There can be multiple steps to convert the power source to the load bus. The multiple switching stages generally lead to higher losses and system cost. Another approach directly converts the incoming low frequency AC to high frequency AC, which is then fed to a transformer. The transformer secondary voltage is then reconstructed to a low frequency output. One of the disadvantages with this type of circuit design is the use of bidirectional semiconductor switches.
While there have been attempts to configure AC-AC converters used for voltage and frequency conversion, there remains continued needs for more efficient designs with improved performance.